The invention relates to a method and apparatus for setting registration in a multicolor printing machine having a number of exposure devices for the digital production of color separations, the actions of setting up and combining the color separations being controlled when printing substrates of a different grade are fed in, in order to correct the registration setting.
Printing colored illustrations, in particular colored images, is carried out, for example, by a number of color separations being printed over one another. These are generally the colors yellow, magenta and cyan, as well as black. If required, special colors are added. By overprinting these colors, all color combinations can be achieved, the quality of the prints depending significantly on the in-register overprinting of the color separations. In the case of digital printing processes, for example, electrostatic printing processes, the maintenance of the registration of the overprint is achieved by the image production devices being controlled such that the color separations meet one another in-register when they are transferred to a printing substrate.
A method and an apparatus for color printing are disclosed by U.S Pat. No. 5,689,757. The teaching of this document is to take account of the influence of the roughness of the paper on the registration when printing substrates of a different grade are fed in. For this purpose, the roughness of a paper, which is to be printed, is measured. The influence of the paper roughness on registration is then taken into account by the registration controller before the paper is fed to the printing process.
It has been shown that influencing the registration accuracy, as a result of changing the printing substrate grade, can be detected only inadequately in this way, so that a high print quality cannot be achieved. The reason for this is that the influence of the change in the substrate grade cannot be detected by a single property. In addition, the paper roughness is, first of all, not the property, which exerts the greatest influence on the registration. This leads to the situation where satisfactory registration accuracy can be achieved only by a correction, which is based on evaluating the printing result. The disadvantage of setting the registration by evaluating the printing result, for example, by registration marks, is, however, that in the event of a change in the printing substrate grade, counteractive control is only possible for the print which follows a print which is faulty because of the change, since the effect of the change in an influencing variable is only detected when it has already caused a registration fault. Changes to the substrate during a print job are, therefore, not possible.
The invention is, therefore, based on the object of a method and an apparatus such that in the event of a change in the printing substrate grade, the influence of this change on registration can be compensated for optimally before the printing substrate is fed to the printing process. According to the invention, the object is achieved with respect to the method and apparatus, in that when the printing substrate grade is changed, the influence of all the properties of the printing substrate that are relevant to registration is taken into account directly as the change is implemented, by correction values for the printing substrate that are available before printing is carried out. At least one controller is provided that when the printing substrate grade is changed, the influence of all the properties of the printing substrate that are relevant to registration is taken into account directly as the change is implemented, by available correction values for the printing substrate.
The invention is based on the finding that there are a number of properties of printing substrates which have an influence on registration, and optimum compensation of the influences of these properties when the printing substrate grade is changed is possible only when the sum of the influences of all the properties that are relevant in this regard is taken into account. If significant properties remain unconsidered, there remains only the subsequent correction, mentioned at the beginning, by evaluation of a proof result.
For instance, it has been shown that a significant influencing variable with regard to registration is the printing substrate thickness. This is because the movements of the elements carrying the color separations and substrates are linked to one another by friction. Here, as a result of a change in the printing substrate thickness, the radius, which is critical for the transmission ratio between the printing substrate carrier and a cylinder, is changed. This changing transmission ratio affects registration to a significantly greater extent than the paper roughness.
However, in addition to the printing substrate thickness and the surface roughness, there are still further properties of a printing substrate which have an influence on registration, such as the compressibility or the flexing of the printing substrate during printing, the stiffness or changes resulting from toner uptake or heating.
The invention achieves an optimum correction, since the sum of all these influences is taken into account, without it mattering which and how many properties of the printing substrate have what influence. In addition, it is possible to include changes in the properties of a printing substrate within the machine, if these properties change from one printing unit to the next as a result of heating, flexing or printing with one or more color separations. For this case, the correction values for the individual printing units are correspondingly different.
The invention, therefore makes, it possible for the occurrence of a registration fault to be prevented from the outset, so that proofs on printing substrates, for example, with registration marks, are normally no longer needed. It is precisely in the case of small jobs or for printing on printed pages with continually changing paper grades that this is of economic importance, since machine time and often also printing substrates can be saved. In addition, the measure according to the invention makes proofs on the printing substrate carriers possible without feeding printing substrates in order to correct other influences on registration, since the influence of the properties of the printing substrates is detected and compensated for in a satisfactory manner by the measure according to the invention.
As a result of the invention, a registration setting of known type can be optimized with the effect that, when a printing substrate grade is changed, it does not xe2x80x9cget out of handxe2x80x9d initially in order to be corrected again later, but rather the registration setting remains within the range of tolerable fluctuations, so that during a change to a different grade of printing substrate, it is possible to continue to print without interruption. At the same time, the invention does not exclude additionally taking into account other influencing variables, which are necessitated by a printed page but not by the printing substrate grade. One example of such an influencing variable is the application of toner, which is determined by the motif in the image and results in a different toner profile with respect to each color separation. In the measure according to the invention, nor does it matter either whether the printing substrate carrier with the printing substrates cooperates directly with the image cylinders in order to transfer the color separations, or whether image transfer cylinders are interposed.
A development of the method provides that in the event of a registration correction because of a change in printing substrate grade, the retroactive influence of the properties of the printing substrate of printed pages which have gone before but are still in the printing process on the registration of the new printed page that is already in the printing process is taken into account. In this case, for each printing unit, account is taken of the fact that the influences on registration are still determined by the printing substrates of the preceding printed pages while the setting of an image on the image cylinder with a color separation for the new printed page is already being performed. Furthermore, it is also taken into account that this change in the printing substrate properties, which have a relevant influence on registration, takes place successively on one printing unit after the other, that is to say, xe2x80x9cruns through the machinexe2x80x9d.
In the described way, the maintenance of registration of the prints in the event of a change to the printed page is still better ensured. The printing machine can also continue to print without any interruption if, between the setting-up of two color separations, the printed-page change takes place in such a way that the new color separation is already being set up while the preceding color separation of the previous printed page has not yet been transferred, or not completely transferred, to the printing substrate. The maintenance of registration is, therefore, ensured to a high degree, and any mutual influence is compensated for, even when different printing substrate grades are printed one after another, even if these change with each printed page. As a result, the economy of the printing machine is significantly increased, and its use for printing individual jobs becomes more efficient.
With regard to the apparatus, this is achieved by the at least one controller being such that in the event of a registration correction because of a change in the printing substrate grade, the retroactive influence of the properties of the printing substrate of the printed pages which have gone before but are still in the printing process on the registration of the new printed page that is already in the printing process is taken into account.
The properties of printing substrates can be taken into account on the basis of stored data for the common printing substrate grades or those that are repeatedly used. A change in the printing substrate grade can be taken into account by storing data on printed pages to be processed. This is recommended in the case of all machines, which, under computer control, process a series of printed pages and different printing substrates, all the essential data on the printed pages, and also the data on the printing substrate to be used, being available. The advantage resides in an automatic procedure that can be implemented by appropriate software.
With regard to the apparatus, in order to take into account the properties of the common printing substrate grades, provision is made for it to be equipped with at least one memory, which contains stored data on correction values of common printing substrate grades or those that are repeatedly used, the at least one controller controlling the actions of setting up and combining the color separations on the basis of this data. Taking account automatically of data about printing substrates is achieved, with regard to the apparatus, by the latter being equipped with a memory to be loaded with data on printed pages to be processed, and the at least one controller determining the necessary correction values from this data. Determining the correction values from the data can be both a direct removal of the same or an access instruction which links the controller with the abovementioned data on printing substrates for common printing substrate grades or those that are repeatedly used.
In addition, it is possible for a new grade of printing substrates to be identified by measuring properties of this printing substrate before the latter is fed to the printing process. With regard to the apparatus, for this purpose a device for measuring the properties of printing substrates is needed, as is a controller, which performs the identification. When measuring properties for identification, these do not have to be properties, which have an influence on registration. Since only one determination of the identity is necessary, light reflection or color could also be used, for example. Further properties such as the paper weight or printing substrate thickness are possible. In addition, a number of properties can be linked for the purpose of unequivocal identification.
Another possibility of determining the correction values for a printing substrate is in correction values for various properties of printing substrates being available as stored data, and in these properties being determined and the correction values resulting from these properties being taken into account. With regard to the apparatus, provision is then made for it to be equipped with at least one memory, which is loaded with data for various properties of printing substrates, for it to have a device for measuring properties of printing substrates, and for the at least one controller to be such that it uses the correction values that result from these properties as a basis for setting registration.
However, given this type of determination of correction values, the influences of various properties, such as the printing substrate thickness, the surface condition, the stiffness and so on, have to be detected separately, in order then to determine the correction values for the respectively determined sum of various properties of an actual printing substrate grade. This procedure is particularly suitable when the printing substrate grades are not common ones or repeating ones.
A new grade of printing substrates can also be taken into account on the basis of a manual input. In this case, the correction values themselves can be input, or it is possible for access to be made back to stored data as a result of the input of an identification. With regard to the apparatus, an input device is provided via which the correction values or the identification can be input.
It is expedient for the correction values for various printing substrates to be machine-specific values based on experience, which can be input manually or advantageously stored in at least one memory belonging to the printing machine. xe2x80x9cMachine-specificxe2x80x9d can relate to a specific machine or to a specific machine type. The values based on experience can be determined by proofs and corrected continually by evaluating prints during continuous operation.
In essential terms, the invention pursues the aim of serving to correct an existing registration control system. The latter can be such that it is based on the detection of registration marks, which are printed by the individual printing units. In this case, the invention provides for this correction to be carried out before a change can have any influence on registration marks. For the apparatus, provision can also be made for the at least one controller to be such that it controls registration by registration marks which are printed by the individual printing units and detected by a registration sensor, and takes the correction values into account before a change in the printing substrate grade can have any influence on registration.
However, the invention can also be used to correct a registration control system, which is based on the detection of the positions of the elements that carry the color separations and substrate. With regard to the apparatus, provision is then made for the at least one controller to be such that it controls the registration by detecting the positions of the elements that carry the color separations and substrate by position detecting elements, and takes the correction values into account before a change can have any influence on the registration marks. The position detecting elements may be rotary encoders.
If the invention is used to correct one of the above-mentioned registration controllers, then a significant advantage of the invention comes into play. Since the invention takes the influence of a printing substrate grade fully into account, the preceding registration setting can be carried out with the machine idling, without any printing substrates. Registration marks can also be detected without any printing substrates, by their being printed directly onto the carrier and then removed again. In this way, no printing substrates are used up for the registration setting.
Of course, provision can be made for further influences on registration resulting from a printed-page change to be taken into account when controlling the registration. This may be, for example, taking into account the toner profiles of the color separations of a specific print. With regard to the apparatus, provision must then be made for at least one controller, which takes these further influences into account, for example the toner profiles.
Of course, provision can also be made for further variables which have an influence on registration because of a printed-page or printing-substrate change to be taken into account when setting registration. These may be, for example, toner profiles, paper grades, the paper stiffness or other influencing variables. With regard to the apparatus, provision must then be made of at least one controller, which takes the further influencing variables into account.
The invention, and its objects and advantages, will become more apparent in the detailed description of the preferred embodiment presented below.